1. Field of the Invention
This invention relates generally to temperature compensation of bandgap voltage references, and more specifically to correction of non-linear output voltage versus temperature errors by generating and applying a correction signal or a superposition of a plurality of correction signals having a second or higher order relationship to temperature, proportional to absolute temperature (PTAT) or complementary to absolute temperature (CTAT).
2. Description of the Related Art
Bandgap references such as that using a Brokaw architecture typically generate an output voltage which is the sum of 1) the voltage drop across a semiconductor junction, having a temperature coefficient complementary to absolute temperature (CTAT), and 2) a voltage having a temperature coefficient proportional to absolute temperature (PTAT); wherein the temperature coefficients of the CTAT and PTAT voltages have approximately the same magnitude but opposite sign. The resulting output voltage is thus relatively stable over a wide range of temperature, since the positive and negative temperature coefficients of the summed voltages cancel. There remains, however, a residual temperature effect on voltage which, in theory, introduces an increasingly negative error as temperature varies either above or below the nominal operating temperature (Tn). Theory predicts second and higher order effects, but terms higher than second order are quite small. The theoretical equation has a T*ln(T) term, and the second order correction compensates for the parabolic term of the Taylor expansion of this T*ln(T) dependency. The resulting voltage versus temperature curve appears to have primarily a parabolic curvature.
Correction circuits have been developed which typically generate a current proportional to the square of temperature, which, when injected at an appropriate node in the bandgap reference circuit, acts to decrease the output voltage error. The current typically generated is PTAT2 (IPTAT2) which increases as the square of temperature. This current is injected into a node of the bandgap reference circuit, generating a correction voltage. When the resulting correction voltage is added to the parabolic uncompensated output voltage, the parabolic curve thus becomes more S-shaped, reducing the output voltage error over a given temperature span.
In some actual integrated bandgap reference circuits, however, the uncompensated voltage versus temperature relationship is not the parabolic curve predicted by theory. Differences in processes, structures, and other variables lead, in many cases, to a voltage having little error above a nominal temperature, but pronounced curvature (voltage error increasing as the square of change in temperature) as temperature decreases from nominal. Applying known compensation to such circuits has a smaller than desired effect on error below Tn, and may increase rather than reduce the error above Tn.
A circuit which will correct the output voltage of a bandgap reference circuit over a wide temperature range is therefore desirable, providing correction in the temperature region or regions needing such correction, in whichever direction is required, and without introducing additional error in a temperature region not needing correction.